Hierarchical data storage

ABSTRACT

A method and system for encoding and recording data in a multi-stream format when the data is initially captured. The multiple streams of data can be combined to provide a high quality, or high resolution, representation, while a reduced subset of the streams can provide a lower quality, or lower resolution, representation. Data storage requirements can be reduced, by deleting one or more of the streams, when there is no longer a need for retention of the data at the higher quality.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present invention claims priority from U.S. Provisional PatentApplication Ser. No. 60/700,329 filed on 19 Jul. 2005, which isincorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates generally to storing data in ahierarchical manner. More particularly, the present invention relates toinitial hierarchical storage using partial data streams thatcollectively form a continuous and complete data stream along withmanagement and retrieval of such digital data, such as in a digitalvideo recorder.

BACKGROUND OF THE INVENTION

Digital recording of data is now commonly used in security andsurveillance applications. Typically, a Digital Video Recorder (DVR)records content, such as video, audio and often other data onto disks orother recording media. The recording is either done continuously or istriggered by specific events.

A DVR has a finite amount of recording space on which the recoded datais stored. Accordingly, the DVR can record for a finite amount of time.To allow for continuous use, the DVR storage space must be madeavailable. To free storage space, the DVR periodically deletes data tomake room for new recordings. Video data, in particular, can consume agreat deal of space, and must be regularly deleted. Deletion of data cancause problems if the data is still needed by a user. To reduce thefrequency of data deletion, and thus allow more data to be recorded, theDVR can capture data at a lower resolution, or can use higher degrees oflossy compression. Both use of a lower resolution and higher lossydegrees of compression result in a reduced quality video stream, whichcan be problematic if a higher quality video stream is required toproperly analyze the data. It is well known, that at most times, only areduced quality data stream is needed, with a higher quality data streamrequired only to capture information about a specific incident or event.One problem is that the higher quality image is often required beforethe event occurs so that an after-the-fact review can make use ofinformation leading up to and following the event. A number of methodshave been used to ensure that data is available when needed, and isavailable at the required quality. The different methods are notmutually exclusive, and are often combined.

The easiest method is to provide more storage space by installing largedisk drives, or multiple drives. While this method is effective, it canalso be costly, in some cases dominating the cost of the DVR.

Another approach is to record selectively. In this case, the DVR recordsonly at “significant” times, such as in response to specific events(e.g. motion or alarms), or at previously scheduled times in the day.Because the DVR is not continuously recording there is a reduction inthe amount of data captured. Thus, the DVR can operate for longerperiods before reaching capacity. The disadvantage of this method isthat the rules must be set up in advance. If the DVR is configured torecord for too short a time after an event, important information maynot be recorded. Alternately, if the DVR is configured to record for toolong a time, the storage capacity issue may not be adequately addressed.As noted above, this also fails to provide any information leading up tothe event. Yet another approach is to adjust the recording parameters.By reducing the quality of the recorded video, by reducing its framerate or by reducing the size of each frame, the volume of data can bereduced significantly. This has the obvious drawback that, while itmight significantly reduce data rates and improve retention time, itmight render the resulting data useless for its intended purpose. Forexample, video of a bank robbery may be of little use if the quality orframe size is not sufficient to recognize a person's face.

It is, therefore, desirable to provide a method and system for storingdata that permits storage and retrieval of high quality recordings,while permitting storage space to be freed up when necessary.

SUMMARY OF THE INVENTION

It is an object of the present invention to obviate or mitigate at leastone disadvantage of previous digital data recording and storage systems,particularly those associated with DVRs.

In a first aspect, the present invention provides a method for recordingreceived data in a digital video recording system, comprising: encodingthe input data to provide at least two digital data streams combinableto produce a high quality representation of the input data; andseparately recording the at least two digital data streams to a storagemedium. In a second aspect, the present invention provides a digitalvideo recorder, comprising: a digital encoder for receiving input data,and outputting at least two digital data streams combinable to produce ahigh quality representation of the input data; and at least two datarecorders for recording the at least two digital data streams to one ormore data storage means.

In a third aspect, the present invention provides a digital video streamembodied within a digital video recording system, the stream including:a first subset of digital data corresponding to a first portion of acomplete digital data stream, the first portion capable ofreconstructing a reduced quality reproduction of the complete digitaldata stream; a second subset of digital data corresponding to a secondportion of the complete digital data stream; and wherein the first andsecond subsets are combinable to reconstruct a high quality reproductionof the complete digital data stream.

Other aspects and features of the present invention will become apparentto those ordinarily skilled in the art upon review of the followingdescription of specific embodiments of the invention in conjunction withthe accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described, by way ofexample only, with reference to the attached Figures.

FIG. 1 is a block diagram of a recording subsystem for a DVR;

FIG. 2 is a first example of a multi-stream recording format accordingto the present invention;

FIG. 3 is a second example of a multi-stream recording format accordingto the present invention;

FIG. 4 is a third example of a multi-stream recording format accordingto the present invention;

FIG. 5 is a fourth example of a multi-stream recording format accordingto the present invention; AND

FIG. 6 is a flowchart illustrating a method of the present invention.

DETAILED DESCRIPTION

Generally, the present invention provides a method and system forencoding and recording data in a multi-stream format when it isinitially captured. The multiple streams of data can be combined toprovide a high quality, or high resolution, representation, while areduced subset of the streams can provide a lower quality, or lowerresolution, representation. Data storage requirements can be reduced, bydeleting one or more of the streams, when there is no longer a need forretention of the data at the higher quality.

A method and system of the present invention provide for encoding a datastream in a hierarchical fashion. A single stream is partitioned into aplurality of streams, each of which are stored separately. At least oneof the stream partitions can be accessed independently of the others,and provides a baseline low quality playback. Higher quality playback isprovided by combining partitioned streams, which can provide multiplelevels of quality based on the number of partitions. Thus, a data streamis stored in a hierarchical fashion by creating partitioned streams fromthe original data stream. The terms hierarchy and hierarchical usedherein are defined to indicate that multiple data streams are formedhaving a range of quality from low (e.g., reduced frame rate) to high(e.g., relatively large frame rate) with possible levels of qualityin-between within a third, fourth, . . . etc., intervening data stream.Including such different data streams within a hierarchical manner makesit possible to free up storage space by the simple expedient of deletingthe files related to one of the different parts, which will result inthe maintenance of an effectively lower quality data stream. In priorart solutions, transforming a high quality data stream to a low qualitydata stream involves an expensive re-processing of the data which isavoided by the method and system presented below.

In the most general sense of the present invention, a data stream issegmented or partitioned into N streams where N is some number greaterthan 1. Each of these N-streams is recorded separately. In oneillustrative embodiment, N=2 such that there are 2 streams of data thatare each recorded separately. However, it should be readily understoodthat more than 2 data streams can be used and thereby recordedseparately. In any case of N streams, a subset of these N streams (e.g.,streams 1 and 3, streams 1 and 2, stream 1 alone, or any combinationwhere N=3) is further used to generate a reduced quality version of theoriginal data. Generally, each subset generates a version of the datastream with a different quality. The permissible subsets are determinedby how the N streams are encoded. For example, in the illustrativeembodiment described herein, one can make use of just stream 1 or acombination of streams 1 and 2. Depending on how the data is encoded, itmay be possible to also make use of stream 2 on its own, although insome embodiments, not every recorded stream can be used independently.

This invention takes advantage of the property of data types that can berepresented in a number of manners and resolutions, allowing a trade offbetween data volume and fidelity of the representation. For example,very high quality video data can be saved at the cost of a great deal ofstorage, or lower quality data can be saved at a reduced frame rate,frame quality and/or picture size, resulting in a need for less storagespace at the expense of lower quality data. The present invention allowsone to store data for whatever period they want (e.g., 90 days) withreduced storage requirements, thereby reducing data storage costs.Similarly, audio can be captured at different sample rates allowingaudio quality to be traded off against storage space. The followingdescription discusses the storage of video data in a DVR system, sincevideo data consumes an overwhelming majority of the storage space, but,as will be understood by those of skill in the art, the same techniquescan be applied to audio and other data as well.

Other data may include temperature readings, pressure readings, flowrates, or any other sensed data that is indicative of some physicalquality or interpretative information. Any sensed data, that can beobtained using high sample rates and that can be archived using a lowersample rate unless an event requiring the higher sample rate, can beused with systems of the present invention. For example, streamed videoin a flammable environment may record video, audio, and temperaturereadings. In accordance with the present invention, high qualitytemperature data with numerous samplings may be retained only for solong as can be reasonably ascertained that a fire did not occur—e.g., amatter of days. Thereafter, the data samplings may be reduced to onlyone reading per hour. Likewise, the video data may only requireretention of high quality data for a similarly short period. Thereafter,the video data may be reduced to a much lower frame rate—e.g., hourlyframes or fewer. In accordance with the present invention, the sampletemperature data is stored as a plurality of data streams that can berecombined to form a high quality data stream. The low resolution datastream can be denoted as stream A, while the additional data that inconjunction with stream A provides the high resolution data can bedenoted as stream B. The first and second streams collectively form thehigh resolution data stream. In this manner, forensic evidence todocument fires can be retained for only the relevant time period when afire would have normally been discovered. For example, if no fires arediscovered within five days from a given moment in time, then it is safeto delete the more complete version of the data stream leaving only thelower quality stream. Thus data stream B is deleted, leaving data streamA in place. In such applications including other data, a DVR systemwould of course include other mechanisms to capture and record dataother than video and audio.

It has been recognized that storing a lower quality data stream requiresless storage space than storing a high quality data stream. However, itis often very costly or inconvenient in terms of data processing totransform the high quality stream to the low one after the fact—i.e.using post processing after initially recording the data. A DVR thatcaptures and records a large number of inputs may not be able to affordthe extra processing and disk I/O requirements of reading previouslyrecorded data, re-processing it and re-recording the result.

The present invention provides a mechanism to mitigate the problems ofre-processing and re-recording by initially encoding the video as ahierarchy of two or more streams. The hierarchy of streams would includea low quality stream and a high quality stream with possibleintermediate streams having a range of quality levels between high andlow. Collectively, the low quality, high quality, and any intermediatestreams would form a single, continuous stream. By combining all streamstogether video data of a very high quality can be reconstructed. Asubset of the streams can be used to reproduce lower quality video. Whenthe video data is processed in this manner where the multiple streams ofvarying quality are formed initially, the individual streams can bewritten to separate files, and post-recording quality reduction can beachieved in a much easier manner. Specifically, post-recording qualityreduction may occur simply by deleting files. Deletion does not add anyprocessing complexity as deletion is an operation that the DVR must doin any event as part of its normal data purging. If more than twostreams are stored, the deletion of stream to reduce the video qualitycan be staggered so that the video quality is intentionally degradedover time as it becomes less important.

FIG. 1 shows a recording subsystem for a digital data recorder 8 such asa DVR. A digital encoder 10 receives an input signal such as an analogvideo signal, digitizes the signal, and outputs two or more streams ofvideo data to data stream recorders 12 and 14. The recorders 12, 14store the data streams A and B to suitable digital data storage medium16, such as a hard drive, CD-R, tape or other medium. Data streams A andB are preferably compressed, either during encoding or during therecording steps. While FIG. 1 illustrates two streams, it should bereadily understood that any of number of data streams and recorders canbe used without straying from the intended scope of the presentinvention. The manner in which the data is recorded to the storagemedium 16 is not critical, provided the recorded information issufficient to allow the two (or more) recorded streams to be merged. Forexample, a typical DVR records the time each frame is captured, alongwith the frame. One way of merging two streams is to take the filesrepresenting the recordings, and present the frames from each in correcttime order. In playback, the data streams are ready from storage 16 bydata stream combiner 18 which combines the data streams and provides acombined data stream as an output. Combiner 18 can be integrated withother components in a playback system including a rendering engine.

In accordance with the present invention, the streams are generated suchthat the video information is divided between the streams and meetspredetermined criteria. Such criteria includes that during playback ofthe video it should be possible to combine all streams to provide thehighest quality video, or to use a subset of the streams to providevideo of lower quality. Also, the volume of data in each individualstream is preferably less than if only a single high quality stream wasgenerated. The combined volume for all streams may be greater than asingle high quality stream, though ideally not greatly so.

There are numerous ways of generating multiple video streams dependingon the compression and encoding methods used and the desired propertiesof the streams. The simplest method is depicted in FIG. 2. This diagramshows a sequence of frames where each frame is indicated by the letter Fwith time advancing from left to right. In this example, a sequence offrames is split into two streams where the data stream A is formed bytaking every fifth frame, and data stream B consists of the remainder ofthe frames. This method is most applicable to compression methods, suchas motion JPEG, where frames are encoded individually and do not makedirect reference to preceding frames. In this case, if data streams Aand B are recombined, video can be viewed at a high frame rate. At somelater time after the initial recording, the files storing data stream Bcan be purged, reducing the storage space by approximately 80 percent.Once the second stream data has been purged, video for this time periodis still available, though at a frame rate one fifth of the original.

The scheme shown in FIG. 3 is an alternative that can be used if usingan encoding technique, such as MPEG 4 video compression, where framesmake reference to preceding frames to increase the efficiency ofencoding. In the MPEG 4 standard, there are several frame types defined.I frames are encoded without reference to any other frame and are thusindependent. P frames are encoded relative to a previous frame, either Ior P and contain partial frame information that describes differencesbetween the frame and the related frame. According to the MPEG 4standard, there must be at least one I frame in a validly encoded MPEG-4video stream. FIG. 3 shows two data streams, data stream A 100 and datastream B 102, with the time index increasing from left to right. Thearrows between the frames indicate the frame upon which each framedepends. Data stream A 100 is a regular MPEG 4 stream having an initialI frame 104 and a series of P frames 106 a and 106 b, allowing datastream A to be played back by itself. As only a fraction of therecordable frames are represented in it, the resulting video has a lowframe rate. Data stream B consists of the remainder of the frames.Stream B is encoded entirely as sets of P frames 108 a, 108 b and 108 c,where the first frame in each set references a frame in stream A andsubsequent frames in the set reference other frames in stream B. If bothstreams are used together, one can make use of all frames from bothstreams with a combined frame rate of five times the frame rate of thefirst stream alone. Once a predetermined retention period has beenreached, or other retention criteria met, the files relating to stream Bcan be deleted, freeing up significant disk space. One skilled in theart will appreciate that the rendering of a combined stream requiresthat frame 104 be reloaded at the end of rendering frames 108 a, so thatframe 106 a can be rendered. Similarly, frame 106 a is loaded prior tothe rendering of frame 106 b. This may introduce slight performancedegradation, but in most instances, this will not be noticeable to aviewer.

In FIG. 4, another MPEG example is shown that allows video quality to bechanged after recording, rather than frame rate. In this case, datastream A 110 is a base level stream made up of an initial I frame 114and a subsequent series of P frames 116. These frames can be recordedusing a high degree of lossy compression so that the size of the filesassociated with stream A is reduced while the frame rate is maintained.Data stream B 112 contains enhancement frames E 118, corresponding toeach of the frames in the stream A. Techniques for encoding enhancementlayers are described in the MPEG 4 standard and are well known in theart. When the two streams are combined it is possible to view video at arelatively high quality. Again, some time after recording, the filescontaining the stream B 112 can be deleted leaving a lower quality videostream in stream A, but freeing up storage space. One skilled in the artwill appreciate that it is possible to combine this exemplary embodimentwith other embodiments, so that a multi-level stream structure iscreated, with one stream being a low quality set of frames, anotherstream increasing the frame rate, and another stream of enhancementframes. This can be structured hierarchically to allow use ofcombinations of the streams as desired to obtain a higher quality imagestream.

A further example is depicted in FIG. 5. This example is similar to thatdescribed in relation to FIG. 3, except that stream B 102 can be decodedindependently of stream A 100. The advantage of this method is that,while it still allows the dropping of data volume by deleting stream B102, it also permits data to be reconstructed from either stream. Thus,if the two streams are recorded to different storage devices, thismethod provides some degree of resilience to data loss. If the disk towhich stream A 100 was recorded failed, it would still be possible tomake use of stream B 100. Rendering independence of stream B 102 isachieved by encoding the first frame of the first set of frames 108 d asan I frame. Subsequent sets of frames 108 b and 108 c are identical tothose illustrated in FIG. 1, save that they are dependent upon the lastframe in the previous set as opposed to being dependent upon frames instream A 100. This allows stream B 102 to be independently rendered.

Generally speaking, while it is possible that the second portion B ofthe overall stream might be usable alone, it should be noted that inmany cases B would not be used alone. In one DVR implementation, eitherthe first portion A or both first and second (A and B) together areused. In such case, the first portion might provide a video at tenframes per second (fps), whereas the combination might provide video at24 fps. Alternatively, a high compression version can be stored in a thefirst video stream, with enhancement information reducing the lossycompression ratio in the second data stream. However, this concurs withthe underlying premise of the present invention whereby the data issplit into multiple streams such that combined they provide fullquality, but a subset (though not necessarily just any subset) can beused to give a reduced quality rendition.

While the above examples encode the data as two streams, the presentmethods can be used to record and display any number of streams. Forexample, frames can be divided between three streams, allowing framerates to be dropped in two steps.

Once the multi-stream video data has been generated and recorded, thereare many ways in which it can be maintained. A DVR can automaticallydelete the data for one of the streams after a fixed number of days. Forexample, high quality video could be made available for 10 days, andlower quality video for another 60 days. No post-processing of therecorded data is required; instead, the files relating to thediscardable stream are simply deleted or archived. To provide morecontrol to a user, the DVR can also permit the user to indicate whendata, at either high or low quality, should be deleted, or retainedpermanently, such as for evidence purposes.

Alternately, the DVR can use the presence of events, such as alarms andmotion, to trigger it to keep both streams of video for two minuteseither side of the event, and just the low frame rate stream for afurther 5 minutes. This keeps the video that is most likely to beimportant at maximum quality while keeping some additional video atreduced quality.

It is also possible to subject audio data to a similar process as video.For example, if audio is captured as a stereo signal, it can be encodedas two streams, a mono stream and one of the original left or rightchannels. From these two streams it would be possible to reconstruct theoriginal stereo signal or one could delete half the data and be leftwith a mono signal at half the data rate.

This invention provides a means by which data can be captured withmaximum quality for use in the short term, but can later be efficientlyreduced in size and quality, to achieve desired retention requirements.This provides a significant benefit over initially recording onlyspecific events or capturing at lower quality, since the decision ofwhat to keep and at what quality to keep it can be made with theassistance of a person who is more likely to know the importance of thedata.

Normally recording at high quality and reducing the quality over timerequires that both high and low quality data is captured initially andthe high quality data discarded first, or that only high quality data iscaptured initially and it is re-processed at a later time to reduce itssize and quality. This invention improves over these alternatives by notrequiring redundant storage of essentially the same information or theexpensive re-processing of existing data.

One skilled in the art will appreciate that in place of a digital videorecorder, any digital data recorder, such as a digital audio recorder ora digital temperature recorder can be used without departing from thescope of the present invention. Digital data recorders can employencoders to encode the digital data in partitioned data streams so thateach stream can be recorded by a recorder and stored. In a playbackscenario, the data streams can be recombined using reordering techniquesto produce a single data stream that can be displayed or played backaccording to the needs of the user.

FIG. 6 illustrates an exemplary method of the present invention. In step150, a data stream is received. This data stream is encoded into aseries of partitioned data streams in step 152. Each of the partitioneddata streams is recorded to a storage medium in step 154. This providesthe hierarchical data structure used by systems of the presentinvention. To access the data, a subset of the recorded data streams isselected in step 156 and are combined in step 158. The combined datastream is then rendered in step 160 in a manner suitable for the type ofdata recorded (to a display for video data, to a speaker for audio dataetc.)

The above-described embodiments of the present invention are intended tobe examples only. Alterations, modifications and variations may beeffected to the particular embodiments by those of skill in the artwithout departing from the scope of the invention, which is definedsolely by the claims appended hereto.

1. A method for recording and managing received input data in a digitalvideo recording system, comprising: encoding the input data to provideat least two digital data streams combinable to produce a high qualityrepresentation of the input data, the at least two data streamsincluding a first subset of digital data corresponding to a firstportion of a complete digital data stream, the first portion capable ofreconstructing a reduced quality reproduction of the complete digitaldata stream, the at least two data streams including a second subset ofdigital data corresponding to a second portion of the complete digitaldata stream, the second subset being combinable with the first subset toform a reconstructed high quality reproduction of the complete digitaldata stream; separately recording the at least two digital data streamsto a storage medium; and in response to an event, retaining portions ofboth the first and second subsets recorded during a first time periodassociated with the event, and retaining portions of only the firstsubset recorded during a second time period associated with the event.2. The method as claimed in claim 1, further including a step ofcombining each of the at least two the separately recorded digital datastreams together to form a high quality data stream.
 3. The method asclaimed in claim 1 further including a step of forming a reduced qualitydata stream from the first subset.
 4. The method as claimed in claim 3wherein each of the separately recorded digital data streams forms asubset of a single high quality stream that corresponds to the inputdata.
 5. The method as claimed in claim 4, wherein each of the first andsecond subsets of digital data includes a volume of data that is lessthan that of the complete digital data stream that forms the highquality representation of the input data.
 6. The method of claim 1further comprising: deleting portions of the second subset recordedoutside the first time period associated with the event; and deletingportions of the first subset recorded outside both the first and secondtime periods associated with the event.
 7. The method of claim 1 furthercomprising, if more than two streams are stored, deleting streams in astaggered manner so that the video quality is intentionally degradedover time.
 8. The method of claim 1 wherein the first time periodincludes a time at which the event occurred.
 9. The method of claim 8wherein the first time period includes equal duration before and afterthe time at which the event occurred.
 10. The method of claim 1 whereinretaining the portions of the first and second subsets recorded duringthe first and second time periods is performed in response to an alarm.11. The method of claim 1 wherein retaining the portions of the firstand second subsets recorded during the first and second time periods isperformed in response to motion.
 12. The method of claim 1 wherein thesecond portion of the complete digital data stream is capable ofreconstructing an other reduced quality reproduction of the completedigital data stream independent of the first portion.
 13. The method asclaimed in claim 12, further including a step of forming the otherreduced quality data stream from the second subset.
 14. A digital datarecorder (DDR), comprising: a digital encoder for receiving input data,and outputting at least two digital data streams combinable to produce ahigh quality representation of the input data; the at least two datastreams including a first subset of digital data corresponding to afirst portion of a complete digital data stream, the first portioncapable of reconstructing a reduced quality reproduction of the completedigital data stream, the at least two data streams including a secondsubset of digital data corresponding to a second portion of the completedigital data stream, the second subset being combinable with the firstsubset to form a reconstructed high quality reproduction of the completedigital data stream; and at least two data recorders for recording theat least two digital data streams to one or more data storage means, theat least two data recorders arranged to, in response to an event, retainportions of both the first and second subsets recorded during a firsttime period associated with the event, and arranged to retain portionsof only the first subset recorded during a second time period associatedwith the event.
 15. The digital data recorder of claim 14 wherein thereceived input data includes video data and the data recorder is adigital video recorder (DVR).
 16. The DDR as claimed in claim 14,wherein each of the first and second subsets of digital data includes avolume of data that is less than that of the complete digital datastream that forms the high quality representation of the input data. 17.The DDR as claimed in claim 16, wherein each of the first and secondsubsets of digital data is formed within separate files storable on theone or more data storage means, the separate files being independentlycapable of deletion.
 18. The DDR as claimed in claim 17, whereindeletion of the second subset of digital data does not affect capabilityof the first subset to reconstruct a low quality reproduction of thecomplete digital stream that forms the high quality representation ofthe input data.
 19. The DDR as claimed in claim 18, wherein the deletionof the second subset of digital data occurs during normal data purgingby the DDR.
 20. The DDR as claimed in claim 14 wherein the at least twodata recorders are further arranged to delete portions of the secondsubset recorded outside the first time period associated with the event,and to delete portions of the first subset recorded outside both thefirst and second time periods associated with the event.
 21. The DDR asclaimed in claim 14 wherein if more than two streams are stored, the atleast two data recorders are further arranged to delete streams in astaggered manner so that the video quality is intentionally degradedover time.
 22. The DDR as claimed in claim 21 wherein the first timeperiod includes equal duration before and after the time at which theevent occurred.
 23. The DDR as claimed in claim 14 wherein the firsttime period includes a time at which the event occurred.
 24. The DDR asclaimed in claim 14 the portions of the first and second subsetsrecorded during the first and second time periods are retained inresponse to an alarm.
 25. The DDR as claimed in claim 14 the portions ofthe first and second subsets recorded during the first and second timeperiods are retained in response to motion.
 26. The DDR as claimed inclaim 14 wherein the second portion of the complete digital data streamis capable of reconstructing an other reduced quality reproduction ofthe complete digital data stream independent of the first portion. 27.The DDR as claimed in claim 26, wherein one of the at least two datarecorders is arranged to form the other reduced quality data stream fromthe second subset.
 28. A non-transitory machine readable medium storingstatements and instructions for execution by a processor to perform amethod for recording and retaining received input data in a digitalvideo recording system, the method comprising: encoding the input datato provide at least two digital data streams combinable to produce ahigh quality representation of the input data, the at least two datastreams including a first subset of digital data corresponding to afirst portion of a complete digital data stream, the first portioncapable of reconstructing a reduced quality reproduction of the completedigital data stream, the at least two data streams including a secondsubset of digital data corresponding to a second portion of the completedigital data stream, the second subset being combinable with the firstsubset to form a reconstructed high quality reproduction of the completedigital data stream; and separately recording the at least two digitaldata streams to a storage medium; and in response to an event, retainingportions of both the first and second subsets recorded during a firsttime period associated with the event, and retaining portions of onlythe first subset recorded during a second time period associated withthe event.